Mike Huckabee, the former governor of Arkansas and the only candidate with a chance of defeating McCain, subsequently dropped out of the race.

Al Jazeera's Rob Reynolds said Bush's endorsement would help McCain raise funds and his victory gave him a head start on the Democratic nominee.

"He will have plenty of time to consolidate while Democrats are snapping at each other."

Clinton battles back

In the Democratic race, both Hillary Clinton and Barack Obama claimed victories as their bruising battle for their party's nomination seemed set to drag on throughout the summer.

Delegate breakdown

Democrats

Republicans

Clinton - 1,175

McCain - 1,230

Obama - 1,307

Total needed: 2,025

Total needed: 1,191

Source: Reuters

Clinton finally halted the momentum Obama had built up of 12 straight victories by winning the key battleground states of Texas and Ohio.

She also clinched the state of Rhode Island, while Obama won Vermont.

Intriguingly, the New York senator did not rule out the possibility of running a joint campaign with Obama.

"But of course we have to decide who is on the top of the ticket," she told CBS television.

"I think the people of Ohio very clearly said that it should be me."

Superdelegates issue

Obama, however, said he was still in control of the Democratic race and pointed to his lead among delegates pledged to nominate him at the party's convention in August.

"We've won twice as many states, we've won a greater share of the popular vote and, what I think [is] most important, we've won far more independents and Republicans than Senator Clinton has," he told CBS.

"And that's, I think, what we're going to need to do to beat John McCain in the fall."

The Democratic stalemate puts the focus back on more than 350 uncommitted "superdelegates" - elected party officials and insiders - who could heavily influence the race if they back either Obama or Clinton.

The next election primary will be held in the southern state of Mississippi on March 11, while the Democrats are due to hold caucuses on March 8.

The state of Pennsylvania will also hold a primary on April 22.

Meanwhile the Canadian government says it is planning an investigation into the reporting leaking of a memo which implied Obama's campaign had said that his stance towards Nafta was not as protectionist as he had indicated.